Seismic retrofit of underground structure with large opening using beam-end horizontal haunch

Underground structures with large openings (USLO), especially those that allow natural light and fresh air, have emerged as alternatives to mitigate the weaknesses of traditional underground frame-box structures. For the USLO, two ends of the upper-story beam are generally recognised as weakest regions during strong earthquakes; however, insufficient attention has been paid to improving their seismic safety. This study performed a detailed numerical comparison of the conventional USLO and beam-end horizontal haunch retrofitting USLO under different seismic intensities, and evaluated the effectiveness of the proposed retrofitting scheme. The finite element numerical modelling approach was validated against shaking table test results, where the numerical results were in good agreement with measured data. Based on the validated numerical methods, the two ends of the upper-story beam in the conventional USLO were strengthened with horizontal haunches. Both soil-structure systems were excited by equal earthquake loads. Various seismic responses were compared between the conventional and retrofitted USLO, including structural strain, tensile damage, and story drift. Numerical simulation results indicate that beam-end horizontal haunch retrofitting significantly reduces the tensile strain and maximum damage degree at the ends of the upper-story beam, as well as the upper-story drift, without changing the lower-story drift. Therefore, beam-end horizontal haunch retrofitting is a potentially effective measure for improving the seismic performance of the USLO.